1. Technical Field
The present invention relates to a vibrator element, a vibrator, an oscillator, and an electronic device, and more particularly, to a vibrator element that excites flexural vibration and a vibrator, an oscillator, and an electronic device each having the vibrator element.
2. Related Art
In a vibrator element that excites flexural vibration, a temperature difference occurs between a compressed portion in which compressive stress of a vibrating arm acts and an expanded portion in which tensile stress of the vibrating arm acts. The vibrator element loses vibration energy due to thermal conduction which occurs in order to reduce this temperature difference. A decrease in the Q value caused by the thermal conduction is referred to as a thermoelastic loss effect (hereinafter referred to as thermoelastic loss). Therefore, it is necessary to design the vibrator element considering the thermoelastic loss. For example, according to the technique disclosed in JP-UM-A-2-32229, for example, the Q value indicating the stability of resonance can be improved by forming grooves in the arm portion of a crystal vibrator.
However, when the vibrator element is miniaturized and made thin in accordance with the miniaturization and reduction in the thickness of electronic devices, it is very difficult to form grooves in the vibrating portion with high accuracy.
In order to solve such a problem, JP-A-2009-5022, JP-A-2009-5023, and JP-A-2009-5024 disclose a vibrator element in which a vibrating portion is made thin and a piezoelectric layer is formed on the vibrating portion. In the vibrator element having such a configuration, the vibrator element can be excited to vibrate in a direction (normal direction) crossing the formation surface of the piezoelectric layer by applying an electric field with different potentials to the front and rear surfaces of the piezoelectric layer.
Moreover, JP-A-2005-331485 discloses a vibrator element in which a vibrating arm has a curved surface shape so as to form a concave shape on the electrode side in an initial standby state where no dynamic external force is applied, thereby improving the alignment properties of the piezoelectric layer and obtaining high piezoelectric efficiency.
In recent years where the size and thickness have been decreasing further, the cavity in a package that mounts the vibrator element therein has also been narrowing. Therefore, in the vibrator elements having the vibrating portion that performs flexural vibration in the normal direction as disclosed in JP-A-2009-5022, JP-A-2009-5023, JP-A-2009-5024, and JP-A-2005-331485, there is a possibility that the tip end of the vibrating portion makes contact with the lid of the package when it receives an impact during oscillation or when dropped. Particularly, in the vibrator element having the configuration as disclosed in JP-A-2005-331485, the possibility of contacting is high, which makes it difficult to decrease the height of a vibrator.